Ok i ahve a 94 z28 with a little internal work and free power adders and chips.. I can drive a consistant speed for 2 or 3 miles my service engine light will flash on and fade off and sometimes come on and soemtimes stay on until i accelerate or slow down any idea why my service engine soon light will do this..?

 

One possibility is an electrical problem (solenoid or connection) when the PCM tries to operate the EGR.

 

A scan will tell you what the light is about. Probably EGR as it is one that can come and go during cruise.